In present-day magnetic resonance tomography (MRT), or nuclear spin tomography as it is also known, high voltages, field strengths and gradients, i.e. spatial changes, in particular in the magnetic flux density B, and rapid changes with respect to time, i.e. rates of change, come into play. At the same time, implants, in particular of types comprising metallic components, are in increasingly widespread use. In order to examine patients fitted with an implant, in particular with an active implant, such as, for example, a cardiac pacemaker, a defibrillator or a neurostimulator, in a nuclear spin tomography machine, i.e. in an MRT system, the implants must on the one hand be suitable for an examination of the type, i.e. for use in an MRT system, and on the other hand the MRT system must not lead to the patient, the implant or a function of the implant being put at risk or provoke correspondingly hazardous conditions. Potential risks or unwanted effects are just as likely to reside in a defect, a failure or a malfunction of the implant, for example due to induced voltages, electrical or magnetic fields, as in a causing of burns to the patient due to the heating-up of the implant during the examination. Depending on the type of implant, it is also possible in certain circumstances for cardiac arrhythmias to occur as a result of a nerve stimulation, since the implant can constitute an antenna for electrical and magnetic fields.
Although modern-day MRT systems are generally very reliable and safe in operation, they are at the same time characterized by an enormous complexity. Furthermore, users themselves can set parameters of the MRT system that are used in a measurement sequence and/or can even program measurement sequences. Comprehensive additional protective measures are therefore necessary in order to satisfy current requirements relating to patient safety. This can lead for example to certain functionalities being configured with full redundancy, involving huge overheads in terms of components, manufacturing processes and costs. For example, two actual values of a coil current could be determined and provided independently of one another, and provision could be made for two independent monitoring circuits, for example DSP (Digital Signal Processing) modules, in a system controller and for two separate independent shutdown routes or shutdown paths.
DE 198 57 525 A1 discloses a power amplifier, in particular a gradient amplifier of a nuclear spin tomography apparatus, comprising a switched output stage and a pulse width modulator. In order to avoid short-circuit driving actions in the output stage, a clock generator and a plurality of safety time circuits may be provided in the power amplifier. Thus, if a soft-stop signal is transmitted in order to initiate a shutdown, the corresponding safety times are observed, i.e. the shutdown is performed in a correspondingly clocked manner. In response to the soft-stop signal, the switched output stage forms a bypass to a gradient coil, for which purpose, in a corresponding H-bridge, two switches of the switched output stage are turned on or closed and two other switches of the switched output stage are turned off or opened.
DE 10 2016 202 443 B3 describes a circuit arrangement, a gradient amplifier for magnetic resonance imaging, and a method for compensating for nonlinearities.